1. Field of the Invention
The present invention relates in general to oscillators, and more specifically to an overtone crystal oscillator.
2. Description of the Related Art
A phase-locked loop (PLL) is a basic building block in many electronic circuits, including communication systems and the like, for providing requisite operating frequencies. A timing reference, such as a crystal oscillator, provides a reference frequency FREF which is multiplied by a PLL circuit to achieve a higher output frequency FOUT. Crystal oscillators are highly accurate and thus are often used for providing the reference frequency. It is desired to provide a crystal oscillator having its reference frequency (FREF) as high as possible. Higher crystal oscillator reference frequencies allow for lower synthesizer close-in phase noise, lower loop divider ratios, and wider PLL bandwidths. The wider PLL bandwidths allow for lower cost and complexity when supporting wider modulation bandwidths in communications systems.
An overtone crystal oscillator is often used to provide the timing reference at a relatively high frequency, such as 100 megahertz (MHz) or more, while avoiding the cost of an expensive inverted-mesa crystal. An overtone crystal oscillator has a fundamental mode of oscillation which is suppressed to ensure oscillation at a higher harmonic frequency, otherwise called the overtone. In many overtone crystal oscillator configurations, an inductor-capacitor (LC) circuit is used to suppress the fundamental frequency of the overtone crystal. The LC circuit uses an inductor (external or internal), which adds extra material and manufacturing cost, printed circuit board (PCB) space or integrated circuit (IC) area, and potentially enables undesired coupling to or from nearby circuitry. The inductor or “coil” is thus an undesired component.
Existing coil-less solutions employ differential bipolar transistor circuit topologies that provide limited sideband noise performance. Complementary metal oxide semiconductor (CMOS) processes provide limited support for bipolar configurations and bipolar configurations are not supported by the more recent generations of fine-line CMOS-only process technologies. It is desired to provide a high frequency, fully integrated overtone crystal oscillator without an inductor. It is further desired to enable the overtone crystal oscillator to be configured such that it can be implemented in a standard CMOS process flow if desired.